In parallel with an increasing tendency toward the miniaturization of integrated circuits in the electronic industry, there has been a trend aimed at the production of circuit boards which permit high density mounting to take full advantage of the miniaturization of the devices. However, with the movement to higher and higher density, the size of the plated through holes between conductive metal layers in the printed circuit boards must get smaller. The present technology requires hole diameters of under 0.005 inch. Drilling these holes is not possible with the current technology, and maintaining the tight tolerances on the size of the holes has reached the limits of machine capabilities.
Therefore, one of the problems to be addressed by the present invention is to provide a method for obtaining connecting through holes or posts between layers in a multilayer board which are smaller in diameter than 0.005 inch, and in fact reach diameters as low as 0.0015 inch.
A related problem to be addressed by this same invention lies in the different dielectric materials used in the present manufacture of printed wiring boards. At this time, the dielectric material that has the best overall properties is the polyimide. However, there are three basic problems inherent in this material. Polyimide, like all other dielectrics on the market, absorbs moisture from the atmosphere when left in an open area, and also during any wet processing step in the construction of the board. This moisture tends to degrade the material and weaken adhesion between the dielectric and metal layers. Further, polyimide has a dielectric constant of about 3.5 which is sufficient for the present needs. However, the high performance computer board requires a lower dielectric constant to satisfy the high speed signal propagation requirements.
Finally, the present technology requires that a multilayer board be constructed of separate layers of conducting metal and dielectric, and these layers laminated together and appropriate through holes drilled in the surface, and these holes then be plated. After these steps are completed, the board can be tested for functionality. If there is any defect in the board, it cannot be detected until the complete core or the completed printed wiring board has been built. Thus, a further problem to be addressed by this invention is the adoption of a method and structure for a printed wiring board which will allow for easier testing as the assembly of a printed wiring board is progressing.